Technical Abstract: Tall larkspurs (Delphinium spp.) continue to be the most serious cause of cattle losses on mountain rangelands in the western U.S. Over 40 norditerpenoid alkaloids have been reported in species of larkspurs and toxicology data on 25 of these have been reported by us. These alkaloids can be classified into three general types based on their structural characteristics and toxicity: The N-(methylsuccinyl) anthranoyl-lycoctonin (MSAL)-type, having high toxicity; the lycotonine-type, with moderate toxicity; and the 7,8-methylenedioxylycoctonine (MDL)-type, of low toxicity. The structural importance of the methylsuccinimido anthranilic acid ester group at the C18 position is evident in the high toxicity of MSAL alkaloids, particularly methyllycaconitine (MLA), Nudicauline (NUD) and 14-deacetylnudicauline (14-DAN). Other structural aspects of these alkaloids such as the C14 functionality are also important as demonstrated by the reduced toxicity of barbinine. Methyllycaconitine is the alkaloid of most importance in toxicity of larkspurs on mountain rangelands because of its prevalence in most larkspurs and high toxicity. While Nudicauline and 14-deacetylnudicauline also possess high toxicity, they are relatively minor components in few larkspur species (generally the plains and low larkspurs) but when present at concentrations > 1mg/g dry weight they contribute significantly to overall toxicity. Delta line is often found in high concentrations in many larkspurs but because of low toxicity, its contribution to larkspur poisoning in the field is relatively minor and it will probably not cause toxicosis in the absence of the MSAL-type alkaloids.